Holographic recording medium and holographic recording medium manufacturing method

ABSTRACT

A holographic recording medium includes a transparent layer having a servo surface and a light incidence surface which face each other; a reflecting layer formed on the servo surface-side transparent layer; and a holographic recording layer formed on the light illuminating-side transparent layer. The transparent layer includes resin films laminated so that adjacent resin films differ in stretching direction by 90 degrees.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2003-185274 filed on Jun. 27,2003 the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1) Field of the Invention

The present invention relates to a holographic recording medium and amethod of manufacturing the holographic recording medium.

2) Description of the Related Art

Recently, collinear holographic recording systems in reflection geometryhave been proposed to solve disadvantages in the angular multiplexing inthe transmission geometry (See, for example, Japanese Patent ApplicationLaid-open No. 2002-123949). An optical recording medium used for suchsystems includes a plastic substrate provided at least below a recordinglayer, and a reflecting layer on its rear surface. A recording beam anda reference beam are passed through a coaxial optical path and incidenton the recording layer of the optical recording medium. An interferencepattern is formed on this recording layer by interference of therecording beam incident on the recording layer with the reference beamtransmitted by the substrate, reflected by the reflecting layer, andtransmitted again by the plastic substrate below the recording layer. Atthe same time, an interference pattern is formed thereon by theinterference of the reference beam incident on the recording layer withthe recording beam transmitted by the recording layer and the plasticsubstrate below the recording layer, reflected by the reflecting layer,and transmitted again by the plastic substrate below the recordinglayer. Therefore, if the plastic substrate shows birefringent, rotarypolarization occurs to each of the recording beam and the reference beamafter each light is transmitted by the plastic substrate. As a result, amismatching occurs between a plane of polarization of the recording beamand that of the reference beam, thereby disadvantageously reducingamplitudes of the interference patterns. The amplitude reduction of theinterference patterns causes a reduction in a signal-to-noise ratio (SNratio, a ratio of an intensity of the signal light to a noise) inrecording.

Meanwhile, development of a low birefringent plastic substrate to beused in a currently available optical disk is underway. However, abirefringence of the low birefringent plastic is not low enough.Therefore, even if the low birefringent plastic substrate is used as thesubstrate of the holographic recording medium, the mismatching occursagain between the plane of polarization of the recording beam and thatof the reference beam. As a result, the recorded light cannot be oftenreproduced and the SN ratio is reduced.

SUMMARY OF THE INVENTION

It is an object of the present invention to at least solve the problemsin the conventional technology.

A holographic recording medium according to one aspect of the presentinvention includes a transparent layer including a servo surface and alight incidence surface facing each other; a reflecting layer located ona servo surface-side of the transparent layer; and a holographicrecording layer located on a light incidence-side of the transparentsubstrate. The transparent layer includes resin films laminated,adjacent resin films differing in stretching direction by 90 degrees.

A holographic recording medium according to another aspect of thepresent invention includes a transparent layer including a servo surfaceand a light incidence surface facing each other; a reflecting layerlocated on a servo surface-side of the transparent layer; and aholographic recording layer located on a light incidence-side of thetransparent layer. The transparent layer includes a mixture of a firstmaterial having a positive specific birefringence and a second materialhaving a negative specific birefringence, at least one of the firstmaterial and the second material being a polymer.

A method of manufacturing a holographic recording medium according tostill another aspect of the present invention includes stretching resinfilms; laminating the stretched resin films so that adjacent resin filmsdiffer in stretching direction by 90 degrees, to obtain a transparentsheet; forming tracking grooves on one surface of the transparent sheet;forming a reflecting layer on the one surface of the transparent sheet;and forming a holographic recording layer on another surface of thetransparent sheet.

The other objects, features, and advantages of the present invention arespecifically set forth in or will become apparent from the followingdetailed description of the invention when read in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a holographic recording mediumaccording to one embodiment of the present invention;

FIG. 2 shows the concept of the present invention;

FIG. 3 is a cross-sectional view of a holographic recording mediumaccording to another embodiment of the present invention;

FIG. 4 is a cross-sectional view of a holographic recording mediumaccording to still another embodiment of the present invention;

FIG. 5 is a cross-sectional view of a holographic recording mediumaccording to still another embodiment of the present invention;

FIG. 6 is a cross-sectional view of a holographic recording mediumaccording to still another embodiment of the present invention;

FIG. 7 is a cross-sectional view of a holographic recording mediumaccording to still another embodiment of the present invention;

FIG. 8 is a cross-sectional view of a holographic recording mediumaccording to still another embodiment of the present invention;

FIG. 9 is a cross-sectional view of a forming method of holographicrecording medium according to one embodiment of the present invention;

FIG. 10 is a schematic diagram of one example of the method forrecording information on the holographic recording medium;

FIG. 11 is a schematic diagram of one example of the method forreproducing the information recorded on the holographic recordingmedium; and

FIG. 12 is a schematic diagram of one example of a recording andreproduction apparatus capable of carrying out the recording methodaccording to FIG. 10, and the reproduction method according to FIG. 11.

DETAILED DESCRIPTION

Exemplary embodiments of the present invention will be explained indetail with reference to the accompanying drawings.

In the embodiments of the present invention, a transparent layer whichenables polarization matching between a recording beam and a referencebeam in a recording layer, on which information is recorded as hologramsby changing a refractive index of the recording layer, is arranged onthe reflecting layer-side recording layer.

FIG. 1 is a cross-sectional view of a holographic recording mediumaccording to one embodiment of the present invention. In the holographicrecording medium 10 shown in FIG. 1, a reflecting layer 12 and apassivation layer 11 for the reflecting layer 12 are sequentially formedon a servo surface 13-side transparent layer 14. A recording layer 15and a passivation layer 16 for the recording layer 15 are sequentiallyformed on a light incidence-side transparent layer 14.

The transparent layer 14 is manufactured by laminating a plurality ofstretched and oriented resin films so that the resin films differ instretching direction by 90 degrees. Specifically, the transparent layer14 includes a resin film 14 a stretched in an x direction and a resinfilm 14 b stretched in a y direction, whereby the transparent layer 14functions as a transparent substrate. By thus laminating the resin films14 a and 14 b so as to differ in stretching direction by 90 degrees, achange of a light can be returned to a state before the light isincident on the transparent layer 14. FIG. 2 shows this concept. It isassumed that two resin films equal in thickness are laminated so as todiffer in stretching direction by 90 degrees. If the light is passedthrough a first resin film, the light is changed from a linearlypolarized light to an elliptically polarized light. If the light is thenpassed through a second resin film, a phase delay of the light is offsetand the elliptically polarized light returns to the linearly polarizedlight.

If the resin films equal in material are stretched similarly, the phasedelay of the light is proportional to the thicknesses of the resinfilms. It is, therefore, the easiest method for forming the transparentlayer by laminating even-numbered resin films equal in thickness so thatpairs of the resin films differ in stretching direction by 90 degrees,respectively. In principle, it suffices that the paired films differentin stretching direction by 90 degrees are equal in total thickness.

As a material of the resin films 14 a and 14 b, so-called engineeringplastic transparent and having a high mechanical strength can be used.The engineering plastic may be either a polymer having a positivebirefringence by which a refractive index of the polymer is high in anorientation direction, or a polymer having a negative birefringenceopposite to the positive birefringent polymer. Representative polymersinclude polycarbonate resins, polyacrylate, methyl polymethacrylate,polystyrene, poly(ethylenedimethyl acrylate), polydiethyleneglycolbis(allyl carbonate), polyphenylene oxide, polyethyleneterephthalate, MS-resin, AS-resin, poly(cyclohexyl methacrylate),poly(4-methyl-1-pentene), and CR39. The material of the resin films 14 aand 14 b is selected in light of a refractive index and a volume thermalexpansion coefficient of a material of the hologram recording medium. Iftracking grooves are formed in the hologram recording medium bystamping, the material of the resin films 14 a and 14 b is selectedpreferably in light of the tracking grooves as well as the refractiveindex and the volume thermal expansion coefficient of a material of thehologram recording medium.

Each of the resin films to be stretched may contain an additive such asa low molecular weight compound. Such additives may includeplasticizers, crystallits having a birefringent property, and lowmolecular compounds. As the plasticizers, for example, tricresylphosphate, butyl phthalate, dioctyl phthalate, vinyl polymers, andpolyvinyl chloride can be used. As the crystallits having a birefringentproperty, for example, all the crystals except for cubic crystals can beused, and as the low molecular compounds, for example those havingasymmetric carbons and metal complex compounds can be used.

In a step of stretching each resin film, a temperature of the polymer isincreased to a glass transition temperature or higher. At the glasstransition temperature or higher, high molecular chains are plasticallydeformed. The deformation does not occur at a temperature lower than theglass transition temperature. The deformation disappears when a stressis eliminated. This phenomenon is referred to as “stress birefringence”.In the embodiments of the present invention, the transparent layer 14including the resin films having induced orientation and birefringenceis used as the substrate.

The reflecting layer 12 is formed on the servo surface 13 of thetransparent layer 14. A material of the reflecting layer 12 ispreferably a total reflection thin film material relative to operatingwavelengths. If the operating wavelengths are, for example, 400 to 780nanometers, one of Al alloy and Ag alloy is preferably use as thematerial of the reflecting layer 12. If the operating wavelengths are,for example, 650 nanometers or more, one of Au, Cu alloy, TiN and thelike as well as the Al alloy and the Ag alloy. A thickness of thereflecting layer 12 is determined as a thickness which induces totalreflection. The thickness is preferably 50 nanometers or more, morepreferably 100 nanometers or more.

The passivation film 11 consisting of a transparent oxide such as SiO₂or plastic such as polycarbonate having a high mechanical strength isformed on a surface of the reflecting layer 12.

The recording layer 15 is formed on the light incidence-side transparentlayer 14. In this embodiment, the recording layer 15 is an organicrecording layer on which interference patterns can be recorded. Therecording layer 15 is made of a material which undergoes a change mainlyin refractive index by a change that occurs after light irradiation.Examples of the material of the recording layer 15 include (i)photopolymers polymerized by a chemical reaction induced by theirradiation of the light, (ii) organic photorefractive materialsexhibiting a photorefractive effect, and (iii) photochromic materials.

The photopolymers include matrix materials, monomers andphotoinitiators. The photopolymers may further include acid generators,radical generators, coloring matters, oligomers, and reactioninhibitors.

As the matrix materials of the photopolymers, for example various vinylpolymers such as polyvinyl acetate having an ester group, polycarbonatepolyacrylate, norbornene series resins, polymethyl methacrylate,cellulose acetate butyrate, and polystyrene methyl methacrylate can beused. The content of the matrix material is, for example, 20 to 80 wt %relative to the total amount of the photopolymer.

The matrix material may be formed by a polymeric compound which isliquid at an ordinary temperature and a hardening agent for thepolymeric compound. A formation reaction of this matrix occurs whensolutions for forming the recording layer are mixed, and is finishedwhen the recording layer is formed. Examples of the polymeric compoundliquid at the ordinary temperature include epoxy compounds, ethercompounds, ester compounds, and vinyl compounds. To form the matrix, oneor more of these polymeric compounds are used. After the polymericcompounds reacts with the hardening agent, epoxy resin, urethane resin,acrylate resin, urethane acrylate resin, or the like is formed.

The epoxy compounds may include, for example, 1,2,7,8-diepoxyoctane,1,4-bis(2,3-epoxypropoxyperfluoroisopropyl)cyclohexane,3,4-epoxycyclohexylmethyl-3′,4′-epoxycyclohexanecarboxylate,3,4-epoxycyclohexyloxysilane, 1,2,5,6-diepoxy-4,7-methanoperhydroindene,2-(3,4-epoxycyclohexyl)-3′,4′-epoxy-1,3-dioxane-5-spirocyclohexane,1,2-ethylenedioxy-bis(3,4-epoxycyclohexylmethane),4′,5′-epoxy-2′-methylcyclohexylmethyl-4,5-epoxy-2-methylcyclohexanecarboxylate, ethyleneglycol-bis(3,4-epoxycyclohexane carboxylate),bis(3,4-epoxycyclohexylmethyl) adipate, and di-2,3-epoxycyclopentylether. The ether compounds may include, for example, diglycerolpolyglycidyl ether, pentaerythritol polyglycidyl ether, sorbitolpolyglycidyl ether, trimethylolpropane polyglycidyl ether, resorcindiglycidyl ether, 1,6-hexanediol diglycidyl ether, polyethyleneglycoldiglycidyl ether, phenylglycidyl ether, para-tert-butylphenylglycidylether, dibromophenylglycidyl ether, dibromoneopentylglycol diglycidylether, 1,6-dimethylolperfluorohexanediglycidyl ether, and4,4′-bis(2,3-epoxypropoxyperfluoroisopropyl)diphenyl ether. The estercompounds may include, for example, adipic acid diglycidyl ester andorthophthalic acid diglycidyl ester.

The vinyl compounds may include, for example, vinyl-2-chloroethyl ether,vinyl-n-butyl ether, triethyleneglycol divinyl ether,1,4-cyclohexanedimethanol divinyl ether, trimethylolethane trivinylether, and vinylglycidyl ether.

The curing agent may include, for example, primary or secondaryaliphatic amines and aromatic amines, and one or more kinds thereof areused. The curing agent is preferably optically inactive. The aliphaticamines may include for example, ethylenediamine, diethylenetriamine,triethylenetetramine, tetraethylenepentamine, dipropylenediamine,diethylaminopropylamine, hexamethylenediamine, menthenediamine,isophorodiamine, diaminodicyclohexylmethane, and xylenediamine. Thearomatic amines may include, for example, methaphenylenediamine,diaminodiphenylmethane, and diaminodiphenylsulfone.

As the monomers of photopolymers, molecules having an acrylate reactivegroup can be used. Included are, for example, isobonyl acrylate,phenoxyethyl acrylate, diethylene glycol, monoethylether acrylate, andethyl acrylate. Further, vinyl benzoate, vinyl 3,5-dichlorobenzoate, andvinyl 1-naphthoate may be added. In order to increase refractive indexmodulation, acrylates such as 2-naphtho-1-oxyethyl acrylate and2-carbazol-9-ylethyl acrylate may be used. In this case, low refractiveindex acrylates such as (trimethylsilyloxy)diemethylsilylpropyl acrylateand (perfluorocyclohexyl)methyl acrylate may be included. Moreover,N-vinylcarbazole may be added. In addition, polyfunctional acrylatessuch as pentaerythritol triacrylate, trimethylpropane triacrylate,dipentaerythritolpenta-hexaacrylate, ditrimethylolpropane tetraacrylate,and pentaerythritol tetraacrylate may be used. The content of themonomer in the photopolymer is approximately 5 to 50 wt % relative tothe total amount of the photopolymer.

As the photoinitiator for the photopolymer, for example, materials whichcause radical polymerization by irradiation with light and those whichcause cation polymerization. In order to relax the change in volumeduring polymerization, components which diffuse in the reverse directionwith respect to the polymerizable components may be added.Alternatively, compounds having an acid cleavage structure may be addedseparately. The formation of membrane of a medium containing a lowmolecular component requires a liquid-retention structure in the mediumin some cases. Due to cleavage caused by increasing the number ofmolecules in the material, expansion may occur, and there is a mechanismthat constriction induced by the polymerization of monomers iscompensated at least partially by the change in the volume. Thus, thosehaving a small change in volume may be employed.

The photoinitiator for the photopolymer is specifically a materialhaving sensitivity to recording beam. For example,bis(2,6-difluoro-3-pyrrolylphenyl)titanocene,bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide,bis(2,4-cyclopentadien-1-yl-bis(2,6-difluoro-3-(1H-pyrrol-1-yl)-phenyl)-titanium,a mixture of bis(2,6-dimethoxybenzoyl)-2,4,4-trimethyl-pentylphosphineoxide and 1-hydroxy-cyclohexyl-phenyl-ketone,2-methyl-1[4-(methylthio)phenyl]-2-morpholinopropan-1-one, and a radicalgenerator such as2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)butanone-1 can be used.The content of the photoinitiator of the photopolymer can be selectedappropriately depending on the wavelength of recording beam, filmthickness of the recording layer and light absorbing amount of thephotoinitiator, and is for example, approximately 0.1 to 5.0 wt % withrespect to the total amount of the photopolymer.

As the acid generator for the photopolymer, for example, aryldiazoniumsalts, diaryliodonium salts, triarylsulfonium salts, triarylselenoniumsalts, dialkylphenacylsulfonium salts, dialkyl-4-hydroxyphenylsulfoniumsalts, sulfonic acid esters, and iron arene compounds can be used.

The radical generator for the photopolymer includes, for example,aromatic carbonyl compounds, particularlyα,α-dimethoxy-α-phenylacetophenone.

The coloring matter for the photopolymer includes a large number ofsubstances such as azide compounds, 5-nitroacenaphthene,1,2-benzanthraquinone, 1-nitro-4-acetylaminonaphthalene, methylene blue,safranine O, Malachite Green, cyanine dyes, and Rhodamine dyes.

The oligomer for the photopolymer includes, for example, polyfunctionalacrylate resins having a reactive group at both terminal of principalchain of the polymer, and epoxy resins.

The reaction inhibitor for the photopolymer includes, for example, (i) aradical deactivator such as oxygen, (ii) a radical scavenger such asbutylhydroxyanisole, N-tert-butyl-α-phenylnitrone (PBN), and polyphenolcompounds, and (iii) peroxides such as tert-butyl hydroperoxide.

If the auxiliary components such as the coloring matter and the reactioninhibitor are contained in the photopolymer by about 0.1 wt %, a goodeffect can be often expected. If the auxiliary components are containedexcessively, a sensitivity of the recording layer may possibly bedeteriorated. A content of the auxiliary components is, therefore,preferably within about 0.1 wt %.

The photopolymer is obtained by agitating and mixing these materials. Bycasting a resultant mixture, the recording layer 15 can be formed.

If the photorefractive polymer, which is the organic photorefractivematerial, is used as the material of the recording layer 15, therecording layer 15 can be formed by evaporating a solvent of a solutioncontaining components (a charge-generating material, a charge transportmaterial, and a nonlinear optical material) of the photorefractivepolymer. Each component may be either a molecular material or apolymeric material as long as the material exhibits a photorefractiveeffect. The recording layer 15 may be formed by, for example, heatingthe component mixture, and quickly cooling the mixture without using thesolvent.

The charge generating material, as one of the components of thephotorefractive polymer, needs to absorb the recording beam so as togenerate electric charges. However, if a charge generating materialhaving a very high optical density relative to the recording beam isused, the recording beam does not often reach the charge generatingmaterial within the recording layer 15. The optical density (cm⁻¹) ofthe resultant recording layer 15 is, therefore, preferably in a range of10⁻⁵ to ten.

The charge generator includes, for example, (1) phthalocyanine coloringmatters/pigments of metal phthalocyanine, metal-free phthalocyanine, orderivatives thereof, and naphthalocyanine colorants/pigments, (2) azoseries coloring matters/pigments such as monoazo, disazo, and trisazo,(3) perylene series dyes and pigments, (4) indigo series dyes andpigments, (5) quinacridone series dyes and pigments, (6) polycyclicquinone series dyes and pigments such as anthraquinone andanthoanthrone, (7) cyanine series dyes and pigments, (8) charge transfercomplexes comprised of an electronic acceptor and electron-donatingsubstance typified by TTF-TCNQ, (9) azulenium salts, and (10) flarenetypified by C₆₀ and C₇₀ or methanoflarene which is a derivative offlarene. Some of the charge transfer complexes are suitable for thecharge-generating material in this embodiment.

The charge transport material, as one of the components of thephotorefractive polymer, is a material which has a charge transportcapability of transporting holes or electrons. The charge transportmaterial may be a molecular material, a polymeric material, or acopolymer of a plurality of polymers. The charge transporting materialincludes, for example, (1) nitrogen-containing cyclic compounds(heterocyclic compounds) such as indole, carbazole, oxazole, isooxazole,thiazole, imidazole, pyrazole, oxadiazole, pyrazoline, thiadiazole, andtriazole, derivatives thereof, or compounds having them at the principalchain or side chain, (2) hydrazone compounds, (3) triphenylamines, (4)triphenylmethanes, (5) butadienes, (6) stilbens, (7) quinone compoundssuch as anthraquinonediphenoquinone or derivatives thereof, or compoundshaving these at the principal chain or side chain, (8) flarene such asC₆₀ and C₇₀, and derivatives thereof. Further, (1) π-conjugated polymersand oligomers such as polyacetylene, polypyrrole, polythiophene, andpolyaniline, or (2) σ conjugated polymers and oligomers such aspolysilane and polygermane, and (3) polycyclic aromatic compounds suchas anthracene, pyrene, phenanthrene, and coronene.

If the recording layer 15 is an uppermost surface, the passivation layer16 consisting of a transparent material is preferably provided formechanical protection. The passivation layer 16 may consist of a bulkyglass material, a transparent resin material, or a transparent thin filmmaterial. If a film having a high sensitivity photobleaching property ora film having a photochromic property is used as the passivation layer16, it is possible to prevent the holographic recording layer 16 frombeing deteriorated by a natural light, and thereby preferably improve ashelf life of the recording layer 16. Before the recording beam isrecorded on the recording layer 15, the recording layer 15 is in ametastable state in which monomers are dispersed in the recording layer15. In this state, the recording layer 15 is confronted with thedeterioration caused by the natural light. After the recording beam isrecorded, the recording layer 15 is in a stable state in whichpolymerization of the monomers is finished according to an interferencepattern. Therefore, even if the passivation layer 16 is not formed, adisadvantage of shortening of an archival life does not emerge.

Various changes and modifications can be made of the holographicrecording medium according to this embodiment of the present invention.For example, as shown in a holographic recording medium 18 shown in FIG.3, another transparent layer 17 may be arranged between the recordinglayer 15 and the passivation layer 16. Similarly to the transparentlayer 14 used as the substrate, this transparent layer 17 is composed bya laminated body of a plurality of resin films which are laminated so asto differ in stretching direction by 90 degrees. Namely, a resin film 17a is stretched in the x direction, and a resin film 17 b is stretched inthe y direction. If the transparent layer 17 is additionally provided,the SN ratio which is the ratio of the signal light to the noise isimproved.

Alternatively, as shown in a holographic recording medium 20 shown inFIG. 4, an intermediate layer 19 may be arranged between the transparentlayer 14 and the recording layer 15. Further, if the transparent layer17 is provided as shown in FIG. 3, another intermediate layer 19 may bearranged between the transparent layer 17 and the recording layer 15besides the intermediate layer 19 between the transparent layer 14 andthe recording layer 15. By thus providing the intermediate layers 19,diffusion of low molecular weight compounds from the transparent layers14 and 17 to the recording layer 15 and from the recording layer 15 tothe transparent layers 14 and 17 can be suppressed. The intermediatelayer 19 is obtained by forming a film on the transparent layer 14 or 17by sputtering or the like, and the recording layer 15 is formed on theintermediate layer 19.

The intermediate layer 19 can be formed y a material selected from agroup consisting of, for example, Ca—F, MgF₂, CaF₂, PbF₂, BaF₂, Csl,CsBr, MgO, Al₂O₃, BaF₂, ZrF₄, Si—O, Si—N, Al—O, Al—N, Ti—O, Y—O, Zr—O,Zr—N, Cr—O, Ta—O, In—O, ZnO, Sn—O, B—N, Si—C, Ca—F, Zn—S, ZnS—SiO₂,ZrO₂, BaTiO₃, TiO₂, Y₂O₄, CeO₂, Hf, TeO, and diamond. A thickness of theintermediate layer 19 is not limited to a specific one. However, in viewof requirements of not allowing mutual diffusion of matters betweenupper and lower layers, and of sufficiently transmitting the light, thethickness of the intermediate layer 19 is preferably about 10 to 500nanometers.

The number of resin films that constitute each of the transparent layers14 and 17 is not limited to an even number but may be an odd number. Inprinciple, it suffices that the total thicknesses of the respectivepairs of two types of films different in stretching direction by 90degrees are equal. For example, as shown in the holographic recordingmedium 21 shown in FIG. 5, two resin films 14 b (17 a) of 20 micrometersstretched in a certain direction may be prepared, and the two resin film14 a (17 b) of 40 micrometers different in stretching direction fromthat of the resin films 14 b (17 a) may be put between the resin films14 b (17 a). However, the resin films should be equal in stretchingdegree and in birefringence.

Any one of the holographic recording mediums explained above can bemanufactured by forming a sheet by laminating a plurality of resin filmsstretched and oriented so that each pair of resin films differ instretching direction by 90 degrees, forming tracking grooves in thesheet, and cutting the sheet into disks;

FIG. 6 is a cross-sectional view of the configuration of a holographicrecording medium according to another embodiment of the presentinvention.

In a holographic recording medium 23 shown in FIG. 6, a transparentlayer 24 between the reflecting layer 12 and the recording layer 15 canbe formed by injection-molding a mixture of materials each having apositive specific birefringence and materials each having a negativespecific birefringence. This injection molding, in particular, canfurther facilitate obtaining the transparent layer 24 havingzero-birefringence at a lower cost, and selecting the transparent layer24 equal in refractive index to the recording layer 15. The mixture ofthe materials each having the positive specific birefringence and thematerials each having the negative specific birefringence can be formedby either the injection molding or by bonding and cutting of the sheet.The holographic recording medium 23 is equal to the holographicrecording medium 10 shown in FIG. 1 except for use of this transparentlayer 24.

Examples of the material having the positive specific birefringence ofthe transparent layer 24 include polymers, organic crystals, andinorganic crystals. Polymers having positive intrinsic birefringentindex include, for example, polyvinyl chloride, polyethylene,polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE),polychlorotrifluoroethylene (PCTFE), polyphenylene oxide, polycarbonate,polyarylate, and indene derivatives. The polymers having a phenyl groupat the principal chain are polymers having positive intrinsicbirefringent index in most cases.

As the organic crystals; for example, difluoromethane, polyvinylidenefluoride, chlorobenzene, fluorobenzene, aniline, nitrobenzene,nitroaniline, nitropyridine oxide, and dinitroaniline can be used.

As the inorganic crystals, for example, crystals having high anisotropyand high refractive index in the lengthwise direction of the crystalsuch as needle crystals can be used. The crystals having refractiveindex anisotropy include, for example, LiNbO₃, BaTiO₂, SBN, KTN, KNSBN,BSO, BGO, GaAs, InP, and CdTe.

The material having the positive specific birefringence means a materialhaving optical anisotropy. If the material is a polymer, the refractiveindex thereof in a main chain direction is higher than that in adirection orthogonal to the main chain direction. If the material is aneedle-like crystal, the refractive index thereof in a longer crystaldirection is higher than that in a direction orthogonal to the longercrystal direction.

Examples of the polymer having the negative specific birefringenceinclude polymethyl methacrylate (PMMA) and polystyrene (PST). Thematerial having the negative specific birefringence is a material havingoptical anisotropy similarly to the material having the negativespecific birefringence. If the material is a polymer, the refractiveindex thereof in a main chain direction is lower than that in adirection orthogonal to the main chain direction. If the material is aneedle-like crystal, the refractive index thereof in a longer crystaldirection is lower than that in a direction orthogonal to the longercrystal direction. Examples of the needle-like crystal having thenegative specific birefringence include strontium carbonate and sodiumcarbonate.

A mixture ratio of the materials each having the positive specificbirefringence to the materials each having the negative specificbirefringence is selected so that the transparent layer 24 haszero-birefringence. However, the polymer should be selected as thematerial having the positive birefringence or that having the negativespecific birefringences. This is because the transparent layer 24 isrequired to have a high adhesibility to the recording layer 15 accordingto the present invention. The number of materials high in birefringenceΔn per unit volume is set to be smaller than the number of materials lowin birefringence Δn per unit volume.

If the recording medium is disk shaped and recording and reproductionare carried out while rotating the recording medium, the stressbirefringence of the transparent layer 24 should be taken intoconsideration. The stress birefringence means a birefringence thatappears when a material is applied with a stress to thereby extendpolymer chains thereof, and when groups that constitute the chains areoriented. The stress birefringence of the transparent layer 24 accordingto the embodiments of the present invention can be suppressed low. Thisis because a photoelastic coefficient of the transparent layer 24 can besuppressed low. Examples of a combination of materials having low stressbirefringences include PMMA:PVDF=80:20, PMMA:PEO=65:35, and PS:polyphenylene oxide (PPO)=71:29.

Various changes and modifications can be made of the holographicrecording medium 23 shown in FIG. 6. As shown in a holographic recordingmedium 25 shown in FIG. 7, a transparent layer 26 similar to thetransparent layer 24 can be arranged between the recording layer 15 andthe passivation layer 16. As shown in a holographic recording medium 27shown in FIG. 8, intermediate layers 19 may be arranged between therecording layer 15 and the transparent layers 24 and 26, respectively.By arranging the transparent layer 26 and the intermediate layers 19,the effect already explained can be attained.

The holographic recording medium according to this embodiment of thepresent invention can be manufactured by, for example, casing, bonding(roll bonding), or the like. A holographic recording mediummanufacturing method using the roll bonding will be explained withreference to FIGS. 9A to 9C. As shown in FIG. 9A, the recording layer 15is cast on the passivation layer 16, and an adhesive layer 29 is sprayedon the recording layer 15, thereby forming a recording layer sheet 30.Examples of an adhesive include an adhesive using a volatile solvent, ahot melt adhesive using thermoplastic resin, a chemical reaction inducedadhesive, and a photocurable material. As the adhesive used to bond theplastic substrate (transparent substrate) to the intermediate layer 19(consisting of the inorganic compound, metal, or the like) according tothe present invention, epoxy resin, urethane adhesive, the secondgeneration acrylic adhesive, or the like is preferably used.

As shown in FIG. 9B, the reflecting layer 12 and the passivation layer11 are formed on one surface of the transparent layer 14 by sputtering,and the adhesive layer 29 is sprayed onto the other surface of thetransparent layer 14, thereby forming a plastic sheet 31.

A photopolymer sheet 30 is bonded to the plastic sheet 31 via theadhesive layer 29, thereby obtaining the laminating structure shown inFIG. 9C. The sheets 30 and 31 can be bonded by light irradiation,pressurization, or heating. For heating, an ordinary method such as alight irradiation method or a Joule heat application method can be used.

A method for recording information on the holographic recording medium20 shown in FIG. 4, a method for reproducing information recorded on theholographic recording medium 20, a recording and reproduction apparatuscapable of carrying out such recording and reproduction, and a methodfor variously controlling the holographic recording medium 20 will nextbe explained. Although the holographic recording medium 20 shown in FIG.4 is taken as an example, the same recording method, reproductionmethod, and control method can be applied to the other holographicrecording mediums.

FIG. 10 is a schematic diagram of one example of the method forrecording information on the holographic recording medium 20 shown inFIG. 10. In FIG. 10, a solid line 41 denotes the recording beam(S-polarized light), and a broken line 42 denotes the reference beam(P-polarized light).

An incidence system, which the recording beam 41 is incident on,includes a shutter 51 and a spatial light modulator (SLM) 52. By drivingthe SLM 52 to correspond to an information signal, the informationsignal is carried by the recording beam 41. The S-polarized recordingbeam 41 is then incident on a polarization beam splitter (PBS) 53, aprogress direction of which is changed to a direction of the recordingmedium 20 by 90 degrees, and the recording beam 41 is emitted from thePBS 53. The recording beam 41 is then passed through a gyrator 54.

In the example of FIG. 10, the half-split gyrator 54 is set so that aright half rotates a plane of polarization by +45 degrees, and so that aleft half rotates the plane of polarization by −45 degrees. Therefore,in the S-polarized recording beam 41, the plane of polarization of thelight passed through the right half of the gyrator 54 is rotated by +45degrees, and that of the light passed through the left half rotates theplane of rotation by −45 degrees. Thereafter, the recording beam 41 ispassed through an objective 55, and converged on an upper surface of thereflecting layer 12 of the holographic recording medium 20.

The P-polarized reference beam 42 is incident on the PBS 53 from above,and passed straight through the PBS 53. In the reference beam 42, theplane of polarization of the light passed through the right half of thegyrator 54 is rotated by P+45 degrees and that of the light passedthrough the left half is rotated by P−45 degrees. Thereafter, similarlyto the recording beam 41, the reference beam 42 is passed through theobjective 55, and converged on the upper surface of the reflecting layer12 of the holographic recording medium 20.

The recording beam 41 having the plane of polarization at S+45 degreesmatches the reference beam 42 having the plane of polarization at P−45degrees. As typically shown in FIG. 10, therefore, an interferencepattern is formed in the recording layer 15.

In FIG. 10, for brevity, only the interference pattern formed by aninterference between the recording beam 41 having the plane ofpolarization at S+45 degrees and the reference beam 42 having the planeof polarization at P−45 degrees is shown. Actually, the recording beam41 having the plane of polarization at S−45 degrees matches thereference beam 42 having the plane of polarization at P+45 degrees.Therefore, an interference pattern according to the information signalis formed. FIG. 10 also shows a state in which an incident light of therecording beam 41 at S+45 degrees interferes with a reflected light ofthe reference beam 42 at P−45 degrees in a right portion of therecording layer 15. In a left portion of the recording layer 15, anincident light of the recording beam 41 at S+45 degrees similarlyinterferes with a reflected light of the reference beam 42 at P−45degrees. Accordingly, duplex signals resulting from the SLM 52 arerecorded in the recording layer 15.

Since the recording beam 41 and the reference beam 42 are adjusted to beequal in optical path length, a difference between the recording beam 41and the reference beam 42 in optical path is hardly present. Therefore,the information signal from an upper portion of the SLM 52 (incident onthe right half of the gyrator 54) and the information signal from alower portion of the SLM 52 (incident on the left half of the gyrator54) are recorded substantially at the same location of the recordinglayer 15, or recorded left and right of the recording layer 15 once,respectively in the configuration shown in FIG. 10. Since the upper andthe lower portions of the SLM 52 differ in information pattern, thesignals from the SLM 52 are often duplex recorded. However, both theupper and the lower portions of the SLM 52 enable forming equalinterference patterns left and right, respectively. With this method,therefore, a signal quality is not inferior to that of the angularmultiplexing reproduction method in the transmission geometry.

FIG. 11 is a schematic diagram of one example of the method forreproducing the information recorded on the holographic recording medium20.

During reproduction, the shutter 51 located in the incidence system forthe recording beam 41 is closed. It suffices that the shutter 51functions to prevent incidence of the recording beam 41 on the recordingmedium 20. As the shutter 41, a liquid crystal shutter, an S-polarizedlight reflecting plate, or a total reflection plate can be used. Duringreproduction, only the P-polarized reference beam 42 is made incident.

For brevity, FIG. 11 shows the information reproduction method whilepaying attention to the reproduction reference beam 42 incident on aleft part of the PBS 53. The incident P-polarized light is passedthrough the left half of the gyrator 54, thereby rotating the plane ofpolarization of the P-polarized light by P−45 degrees, and the resultantP-polarized light is incident on the recording medium 20 on which theinterference pattern is recorded, through the objective 55. In theexample of FIG. 11, a state in which the reflected light of thereference beam 42 at P−45 degrees is diffracted by the interferencepattern so that FIG. 11 can be compare with FIG. 10. As alreadyexplained in the recording method, the recorded interference pattern isformed by the interference between the recording beam 41 at S+45 degreeswith the reference beam 42 at P−45 degrees. If the reproductionreference beam 42 at P45 degrees is incident on the recording medium 20,the reference beam 42 is diffracted by the interference pattern, andreturned to the objective 55 side. A diffracted light 43 passed throughthe objective 55 and then the gyrator 54 is passed through the gyrator54 in a direction opposite to the direction in which the reference beam42 is incident on the gyrator 54. Therefore, the plane of polarizationof the diffracted light 43 is rotated by +45 degrees. As a result, thediffracted light 43 is transformed to a P-polarized light (at P−45+45degrees), is passed straight through the PBS 53, and reaches areproduction optical system (not shown).

A part of the light which is not diffracted by the interference patternis progressed straight and passed through a right side of the objective55. Since the part of light is passed through the right side of theobjective 55 from below, it is transformed to an S-polarized light (atP−45−45 degrees). Therefore, the light cannot be passed straight throughthe PBS 53 but is bent toward the SLM 52 by 90 degrees. As a result, thepart of the light is not returned to the reproduction optical system anddoes not act as a noise source.

Further, a part of the incident light at P−45 degrees is diffracted byan interference pattern (not shown) stretched on a left side of therecording layer 15 before the light reaches the reflecting layer 12, sothat the part of the incident light contributes to a signal. Namely,both the diffracted light formed by diffracting the reference beam 42reflected by the reflecting layer 12 and the diffracted light formed bydiffracting the reference beam 42 which is not reflected by thereflecting layer 12 contribute to forming a reproduced light. A qualityof the reproduced light is improved, accordingly. In the reproductionreference beam 42, a behavior of the light incident on the right side ofthe PBS 53 is similar to the reproduction reference beam 42 incident asthe polarized light at P−45 degrees on the medium 20 except forincidence on the medium 20 as the polarized light at P+45 degrees.

FIG. 12 is a schematic diagram of one example of a recording andreproduction apparatus capable of carrying out the recording methodaccording to FIG. 10 and the reproduction method according to FIG. 11.

A recording and reproduction apparatus 80 shown in FIG. 12 includes, forexample, a laser light source which outputs a light 40 having a longcoherence length suited for holographic recording as a recording andreproduction light source 57. At present, a most ordinary light sourcefor the holographic recording is a solid laser which emits a laser lightat a wavelength of 532 nanometers. Alternatively, one of a Kr⁺ gaslaser, a semiconductor laser including an external resonator (awavelength of a laser light emitted by which laser can be freelyselected from a blue wavelength to a near-infrared wavelength, typically405, 650, or 780 nanometers), and a semiconductor laser (LD) having along coherence length even if the LD does not include an externalresonator, e.g., a distributed feedback (DFB) laser, a distributed Braggreflector (DBR) laser, or a vertical cavity surface emitting laser(VCSEL) can be used as the laser light source. Depending on a type ofthe laser light source used as the light source 57, a beam formationprism or the like may be provided between the light source 57 and a lens58.

The light 40 emitted from the light source 57 is transformed to aparallel light by the lens 58, and transmitted by a half wave plate 59.An intensity ratio of the recording beam 41 to the reference beam 42 canbe adjusted by rotating the half wave plate 59. Preferably, theS-polarized recording beam 41 and the P-polarized reference beam 42incident on the recording medium 20 are made equal during recording.After the light 40 is passed through the half wave plate 59, the light40 is incident on a PSB 60 and divided into the S-polarized recordingbeam 41 and the P-polarized reference beam 42.

The recording beam 42 is then transmitted by a shutter 51 (not shown inFIG. 12) and an SLM 52, and incident on a half mirror (HM) 61. A PBS ora total reflection mirror may be used in place of the HM 61. Fromviewpoints of an improvement in a utilization efficiency of therecording beam 41, the PBS or the total reflection mirror capable ofreflecting the S-polarized light substantially entirely is preferablyused. From other viewpoints, however, the HM 61 instead of the PBS orthe total reflection mirror is preferably used for the following reason.If the HM 61 is used, a part of the recording beam 41 is made incidenton a photodetector (PD) 62 that detects an information light intensityand the intensity of the recording beam 41 can be detected. Therecording beam 41, an optical path of which is bent by 90 degrees by HM61, is incident on the PBS 53, in which the optical path of therecording beam 41 is bent again by 90 degrees, and incident on therecording medium 20.

The P-polarized reference beam 42 is passed straight through the PBS 60.A part of the P-polarized reference beam 42 is bent by 90 degrees towardthe recording medium 20 by an HM 63, whereas the other part of theP-polarized reference beam 42 is incident on a reference beam PD 64 andused for detection of the intensity of the reference beam 42. Anorientation of the half wave plate 59 can be controlled so as to makethe intensity of the recording beam 41 and that of the reference beam 42incident on the medium 20 equal to each other if the recording beam PD62 and this reference beam PD 64 can detect the intensity of therecording beam 41 and that of the reference beam 42, respectively.

The reference beam 42 bent by the HM 63 toward the medium 20 is passedthrough the PBS 53 and incident on the medium 20. Following this,recording and reproduction are performed according to the methodsexplained with reference to FIGS. 10 and 11.

The reproduction system will be supplementally explained. As alreadyexplained with reference to FIG. 11, the diffracted light 43contributing to reproduction is returned to the P-polarized light andpassed straight through the PBS 53. A part of the diffracted light 43 ispassed straight through the HM 63, and converged on a charge-coupleddevice (CCD) detector 66 by an imaging lens 65. The CCD detector 66converts a light intensity distribution corresponding to an interferencepattern formed in the recording layer 16 into an electric signal,whereby information is reproduced. Further, the other part of thediffracted light 43 passed straight through the PBS is reflected towardthe light source 57 by the HM 63. If a monitor is provided on a frontend or a back end of the light source 57, the diffracted light 43reflected by the HM 63 can be detected by the monitor. By driving thelight source 57 by high frequency superposition, the stability of thelight 40 emitted from the light source 57 can be improved.

A servo optical system will next be explained.

As shown in FIG. 12, in the recording and reproduction apparatus 80, itis normal to provide a servo light source 70 and the recording andreproduction light source 57 independently, and make wavelengthsdifferent from each other. Normally, the wavelength of the light source70 is set longer than that of the light source 57. For example, if thewavelength of the light source 57 is 405 nanometers, the wavelength ofthe light source 70 is set at 532, 650, or 780 nanometers. If thewavelength of the light source 57 is 532 nanometers, the wavelength ofthe light source 70 is set at 650 or 780 nanometers.

If the wavelength of a servo light 44 differs from that of the recordedor reproduced light 40, then the servo light 44 reaches the reflectinglayer 12 in the medium 20 via the PBS 60, the HM 61, and the PBS 53 inthis order, depending on a design of the PBS 60 or the like. Servoinformation is recorded as, for example, pits between the interface(servo surface) between the transparent layer 14 and the reflectinglayer 12. The servo light 44 reflected by the reflecting layer 12,therefore, carries the servo information.

The servo light 44 reflected by the reflecting layer 12 is passedthrough a lens 71, arranged if it is necessary to do so, and detected bya quad-split PD 72 for focusing and tracking. The detected servo light44 is converted into an electric signal, and input to a controller (notshown). Based on an output signal from this controller, an operation ofa voice coil motor (VCM) 73 is controlled to move the objective 55 to anappropriate position. In this way, focusing, tracking, and addressingcontrols are carried out. Alternatively, the servo light 44 reflected bythe servo surface may be divided by arranging a plurality of HM's, andthe focusing, tracking, and addressing controls may be independentlyexecuted using divided servo lights 44, respectively. The configurationof a servo light detection system can be basically made equal to that ofa light detection system used in a digital versatile disk (DVD) drive ora compact disk (CD) drive.

If the focusing, tracking, and addressing controls are carried out, theholographic recording medium 20 is structured, for example, as follows.

If the holographic recording medium 20 is, for example, disk shaped, theservo surface is divided to tracks in a radial direction of the disk andto sectors in a tangential direction. The sector is composed by a headerpart that includes address information, control information, and thelike as a pre-pit pattern, and a data part on which user data can berecorded. For example, in the header part, a pit sequence correspondingto the tracking information and a pit pattern corresponding to theaddress information are sequentially provided to be distanced from eachother along with the shifting direction to the head of the medium 20.Further, in the data part, the surface (servo surface) of thetransparent layer 14 on which the reflecting layer 12 is formed isprovided with no pits and formed as a mirror-finished surface. Namely,if this structure is adopted, tracking is carried out by sample servo.

If irregularities such as tracking guide grooves are formed in the datapart, the recording beam and the reflection light are dispersed by theirregularities, thereby disadvantageously making it difficult to recordand reproduce a desired interference pattern. With the structure adoptedfor the medium 20 explained above, by contrast, this disadvantage doesnot occur since the data part has the mirror-finished surface. It isnoted, however, that it is difficult to make the sample servo trackingto compatible with the CD or the DVD. Considering the low compatibilityof the sample servo tracking therewith, it is advantageous to adoptothers structures for the holographic recording medium 20.

The holographic recording medium 20 has been explained so far, whileconsidering the holographic recording based on the reflection polarizedcollinear holography. If the reflecting layer 12 is removed from therecording medium 20, transmission holographic recording can be alsoperformed.

Information is recorded on the holographic recording medium according tothe embodiments of the present invention by the holographic recordingmethod. In the holographic recording, information is added to one of twolights to make the one light serve as the recording beam, andinterference patterns generated by the interference between therecording beam and the other light (reference beam) are recorded. Due tothis, an optical path difference between the two lights occurs. If alight having a short coherence length, the interference patterns are notgenerated. Therefore, it is preferable to use a laser having a longercoherence length than the optical path difference.

The present invention will be explained in detail with reference toexamples and comparative examples shown below.

FIRST EXAMPLE

An instance in which the recording layer consists of the photorefractivepolymer will be explained.

Each component having the following formulation is dissolved in tolueneto prepare a toluene solution for making a recording layer. Chargegenerating material:  0.2 wt %Diethyl-1,2-methano(60)-flarene-61,61-dicarboxylate Charge transportingmaterial: 30.0 wt %N,N′-Diphenyl-N,N′-(2-naphthyl)-(1,1′-phenyl)-4,4″-diamine Trappingmaterial: 10.0 wt %N,N′-Diphenyl-N,N′-(2-naphthyl)-(p-terphenyl)-4,4″-diamine Non-linearoptical material: 40.0 wt %[[4-(Dimethylamino)phenyl]methylene]-2-methyl- 4-nitrobenzeneaminePolymethyl methacrylate 19.8 wt %

Using the toluene solution, the holographic recording medium shown inFIG. 4 is manufactured.

Two polymethyl methacrylate films each having a thickness of 300micrometers are prepared first, and stretched and oriented at a glasstransition temperature or higher. The two films are laminated so as todiffer in stretching direction by 90 degrees, and a good solvent issprayed and bonded to the laminated films, thereby forming thetransparent layer 14. This transparent layer 14 is used as a transparentsubstrate.

A cylindrical master is stamped on one of the transparent layer 14,thereby forming pregrooves. Ag and SiO₂ are deposited by sputtering,thereby forming the reflecting layer 12 and the passivation layer 11.CaF₂ is deposited by a thickness of 100 nanometers by sputtering on theother surface of the transparent layer 14, thereby forming theintermediate layer 19.

A toluene solution for manufacturing a recording medium is cast on theintermediate layer 19, thereby forming the organic recording layer 15having a thickness of 200 micrometers. Since CaF₂ is insoluble intoluene, the interface between the organic recording layer 15 and theintermediate layer 19 can be made very flat with irregularities of 100nanometers or less. A sheet on which all of these layers are laminatedis cut into a disk, thereby providing a holographic recording medium.

The holographic recording medium thus obtained is evaluated using therecording and reproduction apparatus 80 shown in FIG. 12.

As shown in FIG. 10, a pickup converges and irradiates the recordingbeam 41 and the reference beam 42. In this example, a lens having aneffective aperture ratio of 0.5 is employed, and the laser at awavelength of 532 nanometers and a power of 20 milliwatts is employed asthe light source. By introducing the PBS 53 and the gyrator 54, therecording beam 41 incident on the recording layer 15 interferes with thereference beam 42 passed through the recording layer 15 and reflected bythe reflecting layer 12. At the same time, the reference beam 42incident on the recording layer 15 interferes with the recording beam 41passed through the recording layer 15 and reflected by the reflectinglayer 12.

If a laser beam is converged, a diameter of the beam on the incidentsurface of the organic recording layer 15 is 1200 micrometers and adiameter of the beam on the substrate-side organic recording layer 15 is900 micrometers. After different pieces of information are recorded byshift multiplexing while shifting the beam by 5 micrometers each, thepower of the laser is reduced to a one-hundred and recorded informationis reproduced.

As a result, the recorded information can be reproduced with an accuracyas high as a read after write (RAW) bit error rate of 10⁻⁵ or less.

FIRST COMPARATIVE EXAMPLE

In a first comparative example, a recording medium is manufactured bythe same method as that used in the first example except that apolycarbonate film is used as the transparent substrate.

The recording medium obtained is evaluated by the same recording andreproduction apparatus as that used in the first example. Although theincident light is returned, the RAW bit error rate is 10⁻² or more and agood result cannot be, therefore, obtained.

SECOND EXAMPLE

In a second example, a holographic recording medium is manufactured bythe same method as that used in the first example except that a similartransparent layer is further laminated on the organic recording layer.

The recording medium obtained is evaluated by the same recording andreproduction apparatus as that used in the first example. The RAW biterror rate is not lowered.

SECOND COMPARATIVE EXAMPLE

In a second comparative example, a holographic recording medium ismanufactured by the same method as that used the first comparativeexample except that a similar polycarbonate film is further arranged onthe recording layer.

The recording medium obtained is evaluated by the same recording andreproduction apparatus as that used in the preceding examples. Noreproduced light is observed.

THIRD EXAMPLE

In a third example, a recording layer is formed using the photopolymer.

Each component having the following formulation is blended to prepare aphotopolymer material for making a recording layer. Matrix material:Di(urethane-acrylate) oligomer 63.83 wt % Monomer: Isobornyl acrylate25.0 wt % Vinyl-1-naphthoate 10.0 wt % Photoinitiator: CG-784 1.0 wt %Decoloring agent: tert-Butyl-hydroxy peroxide 0.17 wt %

Using the photopolymer material obtained, a holographic recording mediumin this example is manufactured. The recording medium manufacturedherein is the same as that shown in FIG. 6 except that an intermediatelayer is formed between the transparent layer 24 and the recording layer15.

The photopolymer material is put between Teflon films and partiallycured. Thereafter, the Teflon films are peeled off, thereby forming aphotopolymer film. The photopolymer film has a refractive index of 1.62relative to a light at 532 nanometers. This film is formed to be rolled.

As the material of the transparent layer, PMMA and OVDF are used. ThePMMA and OVDF are mixed together at a weight ratio of 80:20, and aresultant mixture is formed into a disk, thereby forming a transparentdisk. This transparent disk is used as the transparent substrate.

Pre-grooves are formed on one surface of the transparent disk, and areflecting layer is also provided on the one surface thereof. MgF₂ isdeposited on the other surface of the transparent disk by a thickness of0.2 micrometer by sputtering, thereby forming the intermediate layer.

The photopolymer film formed in advance is cut into a disk. Using a hotmelt adhesive, the disk-like photopolymer film is bonded onto theintermediate layer. The adhesive is applied on both the photopolymerfilm and the intermediate layer each by about 1 micrometer by spraycoating. The photopolymer film and the intermediate layer arepress-fitted against each other while irradiating an infrared (IR),whereby the photopolymer film and the intermediate layer can be closelyattached to each other.

The holographic recording medium in this embodiment is therebycompleted.

The holographic recording medium thus obtained is evaluated using thesame recording and reproduction apparatus as that used in the firstexample. In this example, however, the power of the laser is changed to50 milliwatts.

If a laser beam is converged, a diameter of the beam on the incidentsurface of the organic recording layer 15 is 1200 micrometers and adiameter of the beam on the substrate-side organic recording layer 15 is900 micrometers. After different pieces of information are recorded byshift multiplexing while shifting the beam by 3 micrometers each, thepower of the laser is reduced to one-hundredth and recorded informationis reproduced.

As a result, the recorded information can be reproduced with an accuracyas high as the RAW bit error rate of 10⁻⁵ or less.

THIRD COMPARATIVE EXAMPLE

In a third comparative example, a recording medium is manufactured bythe same method as that used in the third example except that apolycarbonate film is used as the transparent substrate.

The recording medium obtained is evaluated by the same recording andreproduction apparatus as that used in the third example. The RAW biterror rate is as low as about 10⁻², and reproduction efficiency is thusquite deteriorated.

FOURTH EXAMPLE

In a fourth example, a holographic recording medium is manufactured bythe same method as that used in the third example except that a similartransparent layer is further laminated on the recording layer.

The recording medium obtained is evaluated by the same recording andreproduction apparatus as that used in the third example. The RAW biterror rate is not lowered.

FOURTH COMPARATIVE EXAMPLE

In a fourth comparative example, a holographic recording medium ismanufactured by the same method as that used in the third comparativeexample except that a similar polycarbonate film is further arranged onthe recording layer.

The recording medium obtained is evaluated by the same recording andreproduction apparatus as that used in the preceding examples. Noreproduced light is observed.

FIFTH EXAMPLE

Two polymethyl methacrylate films each having a thickness of 300micrometers are prepared, and stretched and oriented at the glasstransition temperature or higher. The two films are laminated so as todiffer in stretching direction by 90 degrees, and a good solvent issprayed and bonded to the laminated films, thereby forming a transparentlayer. This transparent layer is used as the transparent substrate.

A photopolymer film equal in composition to that in the third example isformed on the transparent layer to be rolled.

As the material of the transparent layer, PMMA doped with a birefringentinorganic needle-like crystal in a nanosize is used. By orienting thepolymer, particles are also oriented, thereby canceling and eliminatingthe birefringence of the polymer. The crystal used herein is a strontiumcarbonate crystal having a negative birefringence, which is synthesizedby a carbon dioxide gas compounding method. To maintain transparency ofthe polymer, fine particles of a particle diameter of 100 to 200nanometers and an aspect ratio of two to three are used. The material ismolded into a disk, thereby forming a transparent disk. This disk isused as the transparent substrate.

Pre-grooves are formed on one surface of the transparent disk, and areflecting layer is further provided on the one surface. MgF₂ isdeposited on the other surface of the transparent disk by a thickness of0.2 micrometer by sputtering, thereby forming an intermediate layer.

The photopolymer film with the transparent substrate manufactured inadvance is bonded onto the intermediate layer using a hot melt adhesive.The adhesive is applied on both the photopolymer film and theintermediate layer each by about 1 micrometer by spray coating. Thephotopolymer film and the intermediate layer are press-fitted againsteach other while irradiating an IR, whereby the photopolymer film andthe intermediate layer can be closely attached to each other. Thus, thephotopolymer is bonded to the transparent disk. After bonding, the sheetprotruding from the disk is cut, thereby providing a recording medium.

The use of the oriented transparent substrate on the lightincidence-side of the recording layer, and the use of theinjection-molded substrate on the reflecting surface side thereof areappropriate for the recording medium. This is because theinjection-molded substrate has a sufficient mechanical strength, and theoriented transparent substrate can relax a stress applied to therecording layer.

The holographic recording medium in this embodiment is thus completed.

The holographic recording medium thus obtained is evaluated by the samerecording and reproduction apparatus as that used in the third example.

If a laser beam is converged, a diameter of the beam on the incidentsurface of the organic recording layer 15 is 1200 micrometers and adiameter of the beam on the substrate-side organic recording layer 15 is900 micrometers. After different pieces of information are recorded byshift multiplexing while shifting the beam by 3 micrometers each, thepower of the laser is reduced to a one-hundred and recorded informationis reproduced.

As a result, the recorded information can be reproduced with an accuracyas high as a RAW bit error rate of 10⁻⁵ or less.

FIFTH COMPARATIVE EXAMPLE

In a fifth comparative example, a recording medium is manufactured bythe same method as that used in the fifth example except that apolycarbonate film is used as the transparent substrate.

The recording medium obtained is evaluated by the same recording andreproduction apparatus as that used in the third example. The RAW biterror rate is as low as about 10⁻¹, and reproduction efficiency is thusquite deteriorated.

SIXTH EXAMPLE

Two polymethyl methacrylate films each having a thickness of 300micrometers are prepared, and stretched and oriented at a glasstransition temperature or higher. The two films are laminated so as todiffer in stretching direction by 90 degrees, and a good solvent issprayed and bonded to the laminated films, thereby forming a transparentlayer. This transparent layer is arranged on a light incidence-siderelative to the recording layer.

A photopolymer film equal in composition to that in the third example isformed on the transparent layer to be rolled.

As the material of the transparent layer, PMMA doped with a birefringentinorganic needle-like crystal in a nanosize is used. By orienting thepolymer, particles are also oriented, thereby canceling and eliminatingthe birefringence of the polymer. The crystal used herein is a strontiumcarbonate crystal having a negative birefringence, which is synthesizedby a carbon dioxide gas compounding method. To maintain transparency ofthe polymer, fine particles of a particle diameter of 100 to 200nanometers and an aspect ratio of two to three are used. The material isinjection-molded into a disk, thereby forming a transparent disk. Thisdisk is used as the transparent substrate.

Pre-grooves are formed on one surface of the transparent disk, and areflecting layer is further provided on the one surface. MgF₂ isdeposited on the other surface of the transparent disk by a thickness of0.2 micrometer by sputtering, thereby forming an intermediate layer.

The photopolymer film with the transparent substrate manufactured inadvance is bonded onto the intermediate layer using a hot melt adhesive.The adhesive is applied on both the photopolymer film and theintermediate layer each by about 1 micrometer by spray coating. Thephotopolymer film and the intermediate layer are press-fitted againsteach other while irradiating an IR, whereby the photopolymer film andthe intermediate layer can be closely attached to each other. Thus, thephotopolymer is bonded to the transparent disk. After bonding, the sheetprotruding from the disk is cut, thereby providing a recording medium.

The holographic recording medium in this embodiment is thus completed.

The use of the oriented transparent substrate on the lightincidence-side of the recording layer, and the use of the moldedsubstrate on the reflecting surface side thereof are appropriate for therecording medium. This is because the injection-molded substrate has asufficient mechanical strength, and the oriented transparent substratecan relax a stress applied to the recording layer.

The holographic recording medium thus obtained is evaluated by the samerecording and reproduction apparatus as that used in the third example.

If a laser beam is converged, a diameter of the beam on the incidentsurface of the organic recording layer 15 is 1200 micrometers and adiameter of the beam on the substrate-side organic recording layer 15 is900 micrometers. After different pieces of information are recorded byshift multiplexing while shifting the beam by 3 micrometers each, thepower of the laser is reduced to a one-hundred and recorded informationis reproduced.

As a result, the recorded information can be reproduced with an accuracyas high as a RAW bit error rate of 10⁻⁵ or less.

It should be noted that the present invention is not limited to theembodiments. For example, according to the embodiments, the materialobtained by molding a mixture of the materials each having the positivespecific birefringence and the materials each having the negativespecific birefringence, with at least the positive specific birefringentmaterials or the negative specific birefringent materials being polymersis used as the material of the transparent substrate. In addition, thelayer including a plurality of resin films laminated so that respectivepairs of the resin films differ in stretching direction by 90 degrees isused as the transparent layer. Conversely, the layer including aplurality of resin films laminated so that respective pairs of the resinfilms differ in stretching direction by 90 degrees may be used as thetransparent substrate. In addition, the material obtained by molding amixture containing the materials each having the positive specificbirefringence and the materials each having the negative specificbirefringence, with at least the positive specific birefringentmaterials or the negative specific birefringent materials being polymersmay be used as the material of the transparent layer.

According to the embodiments, the step of cutting the reflecting layerand the holographic recording layer besides the transparent sheet afterforming the reflecting layer and the holographic recording layer on thetransparent sheet, is explained as the step of cutting the transparentsheet including a plurality of resin films laminated so that respectivepairs of resin films differ in stretching direction by 90 degrees, intoa disk. However, the present invention is not limited to theembodiments. It is also possible to adopt (i) a step of cutting thetransparent sheet and the reflecting layer into a disk, and forming theholographic recording layer on the transparent sheet cut into the disk,after forming the reflecting layer and the holographic recording layeron the transparent sheet, (ii) a step of cutting the transparent sheetand the holographic recording layer into a disk, and forming thereflecting layer on the transparent sheet cut into the disk, afterforming the holographic recording layer on the transparent sheet, or(iii) a step of forming the reflecting layer and the holographicrecording layer on the transparent sheet cut into a disk after thetransparent sheet is cut into the disk.

Furthermore, the step of forming the tracking grooves on one surface ofthe transparent sheet may be executed either before or after the step ofcutting the transparent sheet into the disk. The transparent sheet canbe cut into a shape (e.g., a square, rectangle, or ellipse) other than adisk at the step of cutting.

As explained so far in detail, the present invention provides theholographic recording medium which is mounted in the holographicrecording apparatus, and which enables recording at the high SN ratiowithout changing the polarization of the recording beam and thereference beam, and the manufacturing method for the holographicrecording medium.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1.-6. (canceled)
 7. A holographic recording medium comprising: a firsttransparent layer including a servo surface and a light incidencesurface facing each other; a reflecting layer located on a servosurface-side of the first transparent layer; and a holographic recordinglayer located on a light incidence-side of the first transparent layer,wherein the first transparent layer includes a mixture of a firstmaterial having a positive birefringence and a second material having anegative birefringence, at least one of the first material and thesecond material being a polymer.
 8. The holographic recording mediumaccording to claim 7, further comprising a second transparent layerlocated on a light incidence-side of the holographic recording layer,wherein the second transparent layer includes a third material having apositive birefringence and a fourth material having a negativebirefringence, at least one of the third material and the fourthmaterial being the polymer.
 9. The holographic recording mediumaccording to claim 8, further comprising an intermediate layer locatedbetween the holographic recording layer and the second transparentlayer.
 10. The holographic recording medium according to claim 7,further comprising a first intermediate layer located between theholographic recording layer and the first transparent layer.
 11. Theholographic recording medium according to claim 10, further comprising asecond transparent layer located on a light incidence-side of theholographic recording layer, wherein the second transparent layerincludes a third material having a positive birefringence and a fourthmaterial having a negative birefringence, at least one of the thirdmaterial and the fourth material being the polymer.
 12. The holographicrecording medium according to claim 11, further comprising a secondintermediate layer located between the holographic recording layer andthe second transparent layer. 13.-17. (canceled)